The increasing cost of fossil fuel and environmental concerns have stimulated worldwide interest in developing alternatives to petroleum-based fuels, chemicals, and other products. Biomass materials are one possible renewable alternative.
Lignocellulosic biomass includes three major components. Cellulose, a primary sugar source for bioconversion processes, includes high molecular weight polymers formed of tightly linked glucose monomers. Hemicellulose, a secondary sugar source, includes shorter polymers formed of various sugars. Lignin includes phenylpropanoic acid moieties polymerized in a complex three dimensional structure. The resulting composition of lignocellulosic biomass is roughly 40-50% cellulose, 20-25% hemicellulose, and 25-35% lignin, by weight percent.
No cost-effective process currently exists for efficiently converting the primary components of biomass, including lignin, to compounds better suited for producing fuels, chemicals, and other products. This is generally because each of the lignin, cellulose and hemicellulose components demand distinct processing conditions, such as temperature, pressure, catalysts, reaction time, etc. in order to effectively break apart its polymer structure. Because of this distinctness, most processes are only able to convert specific fractions of the biomass, such as the cellulose and hemicellulose components, leaving the remaining components behind for additional processing or alternative uses.
Existing methods for converting biomass to usable feedstock are also not sufficient to meet the growing needs. Hot water extraction of hemicelluloses from biomass has been well documented, but the sugars produced by hot water extraction are unstable at high temperatures leading to undesirable decomposition products. Therefore, the temperature range of the water used for hot water extraction is limited, which can reduce the effectiveness of the hot water extraction.
Studies have also shown that it is possible to convert microcrystalline cellulose (MCC) to polyols using hot, compressed water and a hydrogenation catalyst (Fukuoka & Dhepe, 2006; Luo et al., 2007; and Yan et al., 2006). Typical hydrogenation catalysts include ruthenium or platinum supported on carbon or aluminum oxide. However, these studies also show that only low levels of MCC are converted with these catalysts. Selectivity toward desired sugar alcohols is also low. Therefore, a process for converting biomass to polyols for further processing to fuels, chemicals, and other products would be beneficial.
Recent attention has been placed on processes that make use of heterogeneous catalysts to produce liquid fuels and chemicals from biomass. Such processes have the added benefit of being feedstock flexible, continuous and more readily scalable than biological systems involving batch processing. Aqueous-phase reforming (APR) and hydrodeoxygenation (HDO) are catalytic reforming processes that can generate hydrogen, hydrocarbons and other oxygenated molecules from oxygenated hydrocarbons derived from a wide array of biomass. The oxygenated hydrocarbons include starches, mono- and poly-saccharides, sugars, sugar alcohols, etc. Various APR methods and techniques are described in U.S. Pat. Nos. 6,699,457; 6,964,757; 6,964,758; and 7,618,612 (all to Cortright et al., and entitled “Low-Temperature Hydrogen Production from Oxygenated Hydrocarbons”); U.S. Pat. No. 6,953,873 (to Cortright et al., and entitled “Low-Temperature Hydrocarbon Production from Oxygenated Hydrocarbons”); and U.S. Pat. Nos. 7,767,867 and 7,989,664 and U.S. Application Ser. No. 2011/0306804 (all to Cortright, and entitled “Methods and Systems for Generating Polyols”). Various APR and HDO methods and techniques are described in U.S. Patent Application Ser. Nos. 2008/0216391; 2008/0300434; and 2008/0300435 (all to Cortright and Blommel, and entitled “Synthesis of Liquid Fuels and Chemicals from Oxygenated Hydrocarbons”); U.S. Patent Application Ser. No. 2009/0211942 (to Cortright, and entitled “Catalysts and Methods for Reforming Oxygenated Compounds”); U.S. Patent Application Ser. No. 2010/0076233 (to Cortright et al., and entitled “Synthesis of Liquid Fuels from Biomass”); International Patent Application No. PCT/US2008/056330 (to Cortright and Blommel, and entitled “Synthesis of Liquid Fuels and Chemicals from Oxygenated Hydrocarbons”); and commonly owned co-pending International Patent Application No. PCT/US2006/048030 (to Cortright et al., and entitled “Catalyst and Methods for Reforming Oxygenated Compounds”), all of which are incorporated herein by reference.
Similar to petroleum refining systems, biomass catalytic conversion processes require certain processing steps to maintain the effectiveness of the catalyst. Carbonaceous deposits build up on the catalyst surface through minor side reactions of the biomass and other generated products. As these deposits accumulate, access to the catalytic sites on the surface become restricted and the catalyst performance declines, resulting in lower conversion and yields. As a result, process steps are required to remove the deposits and return the catalyst to its desired level of functionality.
A need exists for systems that convert biomass to oxygenated compounds suitable for bioreforming processes, such as APR and HDO. Ideally, the system would convert in a continuous process most if not all of the biomass to compounds, such as starches, saccharides, sugars, sugar alcohols, and other oxygenated products, which are desirable feedstock for bioreforming processes. The system would also allow for operation in either a batch or continuous manner, and provide for the ability to regenerate catalyst without significant interruption to the conversion process.